An important consideration in the design of a computer system is the cooling of the various components within the system cavity. As these components consume electricity in order to perform their desired operations, this electricity is primarily converted into heat through resistive processes. This heat must then be dissipated, otherwise the temperature of the various components will start to rise. If the temperature increase goes beyond the specified operating range of the system and/or its components, then some form of malfunction is likely to occur.
One component of a computing system that is particularly vulnerable to overheating is the CPU chip. This is because its power density tends to be higher than other components within the system. For example, a typical computer system may generate perhaps 100 W of heating power, with some 50% of this arising from the CPU itself. Consequently, the CPU may be producing some 50 W of heating power, despite usually having no more than a few square centimetres in surface area through which to cool. (It will be appreciated that quantitative numbers such as these are presented herein purely for the purposes of illustration, and will vary from system to system and with future hardware development).
A further reason for the relative vulnerability of the CPU chip is its fast signal timing, which generally renders it sensitive to overheating. Thus a CPU chip tends to have not only a high power density, but also a relatively low maximum operating temperature. These factors in combination mean that the CPU is generally the main focus of cooling in computer systems.
Modern computing systems usually rely on a controlled airflow to cool the various electronic components. The effectiveness of this airflow is primarily determined by two parameters, its volume or rate of flow, and its (initial) temperature. As regards the first of these parameters, computing systems almost always use a fan to increase the rate of airflow. Note that the fan can also be used to direct an airflow specifically past those components that are most vulnerable to over-heating (such as the CPU).
It is also important to prevent recirculation of air within a computer system cavity. Thus if such recirculation occurs, then the supposed cooling air will become gradually warmer with each pass over the electronic components, thereby reducing cooling efficiency. Ultimately, of course, if the airflow reaches the same temperature as the electronic components, it will not have any cooling effect at all.
In order to help avoid recirculation, computer systems are usually provided with vented cavities, to allow ambient air to be drawn into the cavity. This incoming air is at room temperature, (hopefully) significantly below the temperature of the electronic components within the cavity, and so provides effective cooling. For example, in a typical system the external packaging of a CPU may have an operational temperature of about 70° C. (the internals of the CPU will be somewhat hotter), and can be adequately cooled provided the incoming ambient air temperature for the computer system is below about 40° C. Of course, the room itself may be provided with air conditioning to maintain the room temperature at a satisfactorily low value.
Although in some circumstances it may be feasible to use passive venting to ensure an adequate ambient airflow into a computer cavity (based on natural convection, for example), in general at least one fan is used to actively drive this airflow. Thus one possibility is to locate a fan adjacent a vent so that it drives air through the vent and out of the computer system cavity. This will then automatically suck fresh room air into the cavity through other vents. Another possibility is for the fan intake to be positioned adjacent a vent, thereby drawing ambient air into the cavity. This in turn will automatically lead to air exiting the cavity through other vents.
FIG. 1 is an illustration of a typical known layout of a computer system 10, comprising a single cavity in which the various components of the computer system are contained. This configuration is typical, for example, of many personal computers. Two particular components within computer system 10 are shown in FIG. 1. One of these is the CPU 12, which as previously mentioned is normally the most significant source of heat within the cavity, and is usually responsible for some 25-75% of the total heat generated. The other component illustrated in FIG. 1 is a power supply 14, which is usually the second largest source of heat in the cavity. Note that since the power supply is based on analog electronics, it tends in practice to be somewhat more robust against a temperature increase than the miniaturised high-speed digital electronics of the CPU.
In most computer systems, the CPU (and potentially other components that are particularly vulnerable to over-heating) is placed in thermal connection with one or more heat sinks (not explicitly shown in FIG. 1). A heat sink is a structure usually made of metal for good heat conduction and typically provided with fins to maximise cooling area. A heat sink therefore serves to efficiently conduct heat away from its associated component(s) and then to dissipate the heat. This helps to prevent an undue temperature rise in the component(s) concerned.
Of course, it will be appreciated that computer system 10 will normally include additional components, such as a hard disk drive, memory, adapter cards, and so on (not shown in FIG. 1). In general, these have relatively minor cooling requirements, since the amount of heat that they generate is comparatively low.
In order to obtain the desired cooling within computer system 10, two fans are provided. A first of these fans 22 is located adjacent CPU 12. The primary purpose of fan 22 is to generate a high-speed cooling airflow in the direction of arrow 25C over the CPU 12, more particularly, in most cases, over the metal fins of its heat sink. Indeed, in some systems fan 22 and the CPU heat sink are provided in effect as a single unit, with the fan located on top of the heat sink and blowing air down into it.
However, fan 22 is unlikely by itself to be able to generate sufficient cooling airflow through or adjacent the other components within computer system 10, such as power supply 14. In addition, the use of fan 22 by itself may result in recirculation of air within computer system 10, rather than a through-flow of air from the external environment. This is because neither the intake nor the output of the fan is clearly directed through a cavity vent, both of these instead being located somewhat centrally in the cavity.
Consequently, computer system 10 is provided with a second fan 24, which is located on the rear wall 18 of the computer system. Fan 24 is operable to expel air out of the cavity of computer system 10 through suitable vents in the rear wall 18. This expelled airflow is indicated in FIG. 1 by arrow 25A. The loss of air from within the cavity of computer system 10 caused by fan 24 then has the effect of sucking in replacement air through vents provided in the front wall 19 of computer system 10. This replacement airflow is indicated in FIG. 1 by arrows 25B.
As a result, there is a general airflow through the cavity of computer system 10 from the front wall 19 through to and out of the back wall 18. This airflow is effective to cool all of the remaining components within the cavity, although by itself would normally not be sufficient to cool CPU 12 (hence the additional provision of fan 22). Note also that fan 24 is positioned approximately adjacent to power supply 14, which is also a relatively substantial generator of heat. Consequently, the air that will be ejected as represented by arrow 25A is primarily drawn past power supply 14, and so provides it with the necessary degree of cooling.
A further technique that is sometimes employed to enhance cooling in computer system cavities is ducting, whereby airflow either into a fan or out from the fan (or both) is guided along a constrained path. Such ducting can be used to direct or concentrate airflow over those components within the system that are particularly vulnerable to overheating. In addition, the ducting can help prevent unwanted recirculation of air within a computer system by connecting either the fan intake or output (or both) to the cavity vents.
FIG. 2 illustrates a computer system 10 that employs a form of ducting (this system is further described in U.S. Pat. No. 6,034,870). As in FIG. 1, computer system 10 includes a CPU 12 and a power supply 14 within a cavity having a front wall 19 and a back wall 18 (again, the other electronic components within the cavity are omitted from FIG. 2 for the sake of clarity). Note that in FIG. 2 there is only a single fan 30, which is placed and orientated so as to direct an airflow primarily towards the two main heat producing components of computer system 10, namely CPU 12 and power supply 14. Since it is the cooling of CPU 12 that is generally the most significant and important for correct operation of the system, the fan is located closest to the CPU, which will therefore receive the strongest cooling airflow (as indicated by arrow 25C). In contrast, power supply 14 is located slightly further away from fan 30, and will generally receive a lesser flow (indicated by arrow 25D), although this should still be adequate for cooling purposes.
Power supply 14 has a vented front wall 14A. In contrast, the side wall 14B of the power supply unit 14 is substantially impermeable to airflow. As a result, the cooling airflow 25D for the power supply 14 passes through wall 14A and is then constrained to travel back through the power supply, before exiting the cavity via vents in the rear wall 18 (as shown by arrows 25A).
Computer system 10 also includes a ducting wall 32. The effect of this is to constrain the airflow 25C between the side wall 14B of the power supply and the ducting wall 32 as it travels past CPU 12, and then exits the cavity, again via vents in the rear wall 18. By constraining airflow 25C in this manner, it is ensured that adequate cooling is provided to CPU 12. In addition, ducting wall 32 serves to deflect some airflow 25E from the fan to the left-most portion of the cavity, where various components (not shown) having generally less stringent cooling requirements can be located. Again, this airflow will exit the cavity through vents in the rear wall 18.
Also shown in FIG. 2 is wall 38, which in effect divides the cavity of system 10 into a fan inflow region, and a fan outflow region. Thus fan 30 draws air in through vents in the front wall 19 of the cavity, as indicated by arrows 25B, and then generates airflows 25C, D, and E, as described above. The use of wall 38 allows fan 30 to be positioned away from the front wall 19 without risk of recirculation, and can also provide certain advantages regarding the input impedance of the fan. Note that the ability to locate fan 30 away from front wall 19 allows it to be positioned in such a manner as to generate airflows 25C, D, and E of the required strength and proportion. It will be also appreciated that these airflows can also be controlled by suitably selecting the size, positioning and orientation of ducting wall 32 (in combination with the fan location, and other properties of the system).
A particular advantage of the configuration of FIG. 2 compared to that of FIG. 1 is that it only requires a single fan. This reflects a general desire to minimise the number of fans within a computer system for several reasons. Thus having multiple fans increases the cost of a system, both in terms of construction and also in terms of operation (the fans consume electricity). In addition, fans produce noise, which can be distracting or irritating for the user of a computer system. Furthermore, and perhaps most importantly, fans are mechanical devices subject to wear and tear, and hence somewhat unreliable compared to the other semiconductor and electronic components of a computer system.
It will be appreciated therefore that the more fans that are included in a computer system, the greater the likelihood that one of them will fail during the lifetime of the system. Such failure will then result in the need to replace the broken fan unit, and there may also be knock-on effects caused by overheating while the fan was not operational (for example, semiconductor components may degrade if allowed to go above their specified operating temperature).
Some systems therefore include multiple fans simply to ensure a level of redundancy, so that even if one fan fails, the remaining fan(s) can still provide sufficient cooling for the system to continue proper operations. This strategy is frequently adopted for a server environment, where it is important to keep system downtime to an absolute minimum. Although this approach does help to mitigate the adverse consequences of fan failure, in other ways it exacerbates the problems by increasing the number of fans in the system. Thus this adds to the expense and running costs, and also increases the risk that system will indeed experience the failure of at least one fan unit during system lifetime. Such systems therefore usually have to be designed to permit relatively easy access to and replacement of a failed fan unit.
Returning now to the configuration of FIG. 2, it will be appreciated that this has only a single fan, and so does successfully minimise the number of fans required. Nevertheless, the arrangement shown in FIG. 2 is still subject to certain deficiencies. Thus the positioning of CPU 12 within the cavity is relatively inflexible. Rather, it must be located in the front central portion of the computer system cavity, in order to ensure that it receives sufficient cooling airflow from fan 30. In addition the degree of cooling in the two front corners of the computer system cavity is rather low, and accordingly these must either remain empty or be occupied by components that do not generate heat, or have only minimal cooling requirements.
Such physical constraints on the layout of system components are becoming more significant, particularly as designers strive to make systems as compact as possible. This is motivated partly by operation at ever increasing frequencies, and with ever increasing bandwidths. For example, a typical processor nowadays has a clock rate of 1 GHz, and the light travel time (in vacuum) for one clock cycle is consequently only 0.3 m. This therefore represents a fundamental limit on the possible physical separation of certain components. (Of course, not all parts of the system are clocked at the same rate).
It is also important for other reasons to try to reduce the lengths of connections between various components. Thus this helps to minimise the loss of analog signal shape and timing as a signal propagates along a connector (caused by various physical properties of the connector itself), and also reduces the risk of picking up interference from stray electromagnetic fields. Moreover, it will be appreciated that the shorter a connector is, the less power is required to transmit a signal through it.
Such concerns mean that there is constant pressure to increase the volumetric density of components in a computer system (especially those not subject to the historical form factors that constrain personal computers). At the same time, it is important to maintain the flexibility of the system designer to optimise the location of the various components within the system, for example to reduce the length of the required inter-component connections, and to maximise the use of available space. This in turn puts greater demands on the cooling system. However, any increase in the number of fans employed is undesirable, in order to avoid degraded reliability and additional costs.